Ball screw assemblies are commonly utilized to translate rotary motion to linear motion or vice-versa. Most ball screw assemblies include an elongate ball screw, and a ball nut body. The ball nut body in cooperation with the ball screw direct a plurality of ball bearings through an internal bearing race formed between the ball nut body and ball screw upon rotation of the ball screw. The ball bearings translate rotary motion of the ball screw to linear motion of the ball nut body. Both the elongate ball screw and ball nut body commonly include a continuous helical groove which defines the internal bearing race.
As is known in the prior art, continuous contact between the ball bearings and that portion of the ball screw forming the bearing race causes significant wear of those parts over time. It is known that the components comprising the bearing race thus must be made from materials that have strength values capable of handling high stresses. Definitive relationships must be maintained between the helical grooves that recirculate the balls through the ball nut body. These relationships are maintained through precision machining and forming operations applicable to the particular high strength material chosen. The materials commonly used in the prior art for manufacturing these components include various metals, typically steel.
Entire ball nut bodies are commonly manufactured from steel and grooves are machined within the ball nut body. Similarly, the elongate ball screw is commonly manufactured from steel including matching helical grooves. The best known methods of manufacturing steel nut bodies incorporating an internal helical groove result in relatively heavy ball nut bodies which are expensive to manufacture. These single piece or unitary ball nut bodies are normally produced by mechanical chip-forming machining starting from a block of steel.
In continuously recirculating ball nut assemblies, either an inner or separate outer ball return or recirculation system must be included to allow the balls to translate properly. Manufacturing processes for forming these recirculation devices on steel nut bodies are generally very expensive. The use of steel ball nut assemblies of the single piece type are thus substantially limited to extremely sophisticated applications in which the need to obtain maximum operational precision and minimum friction outweigh the cost of manufacturing.
U.S. Pat. No. 4,945,781 to Isert discloses a prior art solution to extensive matching of single piece steel ball nut bodies. Isert discloses a ball screw mechanism comprising an elongate cuboid block with gripping jaws for retaining a recirculator. The recirculator has S-shaped, curved, transition grooves for returning the plurality of balls that circulate within the nut, by one flight each on the ball screw. The recirculator is clamped in place between jaws defined on the ball nut body. The ball screw mechanism of Isert thus discloses a ball screw assembly which does not require intricate machining or assembly of an inner or outer ball return system, but is still relatively heavy and inefficient for some applications.
U.S. Pat. No. 3,327,551 to Prueter discloses a ball return means for a ball nut assembly. The ball return mean comprises a ball return tube molded in place inside a rectangular crossover block or guide element. The guide element is made separately from the ball nut member and is free to slide in a longitudinal groove within an internal wall of the ball nut member. The return tube is preferably made of steel and the guide element is preferably made of a bearing grade plastic. As with the Isert reference disclosed above, the Prueter reference discloses a ball nut body which is made of steel and is relatively heavy. Prueter also includes numerous machining and molding steps which would increase the expense of manufacturing such a ball nut.
As stated previously, weight considerations are a factor in choosing a ball nut assembly for use in a particular application. It is known in the aircraft design and defense missile industries, that individual component weight is very important in choosing a component for a particular application. This weight reduction cannot come at the expense of durability, accuracy and increased friction. Thus, it is highly desirable to replace heavy metallic parts with formed polymeric parts if at all possible without a loss in strength and accuracy.
U.S. Pat. No. 4,074,587 to Brusasco discloses a screw-nut transmission coupling utilizing a hardened steel first sleeve having a helical groove defined thereon and a second sleeve formed by direct molding a reinforced plastic or sintered steel onto the first sleeve. The Brusasco reference discloses an internal ball recirculation system utilizing a substantial U-shaped recirculation path formed in the outer surface of the second sleeve. A third metallic sleeve acts as a closing cover for the ball return channel and is mounted on the second sleeve after introducing through the channel a sufficient plurality of balls to completely fill the paths.